Understanding how cardiac myosin regulatory light chain (RLC) phosphorylation alters cardiac muscle mechanics is important because it is often altered in cardiac disease. The effect this protein phosphorylation has on muscle mechanics during a physiological range of shortening velocities, during which the heart generates power and performs work, has not been addressed. We have expressed and phosphorylated recombinant Rattus norvegicus left ventricular RLC. In vitro we have phosphorylated these recombinant species with cardiac myosin light chain kinase and zipper-interacting protein kinase. We compare rat permeabilized cardiac trabeculae, which have undergone exchange with differently phosphorylated RLC species. We were able to enrich trabecular RLC phosphorylation by 40% compared with controls and, in a separate series, lower RLC phosphorylation to 60% of control values. Compared with the trabeculae with a low level of RLC phosphorylation, RLC phosphorylation enrichment increased isometric force by more than 3-fold and peak power output by more than 7-fold and approximately doubled both maximum shortening speed and the shortening velocity that generated peak power. We augmented these measurements by observing increased RLC phosphorylation of human and rat HF samples from endocardial left ventricular homogenate. These results demonstrate the importance of increased RLC phosphorylation in the up-regulation of myocardial performance and suggest that reduced RLC phosphorylation is a key aspect of impaired contractile function in the diseased myocardium. &#8232; We are currently producing full-length cardiac myosin and altering the associated RLC phosphorylation level in-vitro. We are able to determine the RLC phosphorylation level using the Phos-tag molecule for differentiation light chains that are phosphorylated form those that are not in a directly quantifiable gel electrophoresis method. We are using these molecules to run actin-gliding assays to assess the effect of RLC phosphorylation on sliding velocity. We have adapted the NADH-coupled ATPase assay to measure the ATPase rate of full-length cardiac myosin in low salt with different RLC phosphorylation levels. We are twinning these measurements with the three bead optical trap to measure attachment duration, step length and force production by single cardiac full length myosin molecules with different RLC phosphorylation levels.